Battery-powered systems are becoming increasingly ubiquitous for industrial, personal, and medical use. Because of the finite energy storage available to battery-powered systems, battery replacement and/or charging must be performed periodically over the lifetime of these systems. These maintenance procedures typically require physical contact or wire connections with the devices, which may be inconvenient, difficult, or costly in applications such as harsh-environment sensor networks or implanted medical devices. Even where batteries can be easily recharged, the ever-growing hunger for portable power has presented an un-ignorable technical challenge. With the proliferation of wireless devices, the power cable acts as a virtual umbilical cord and presents a cumbersome hindrance to a completely unfettered wireless world.
Recently, numerous solutions have been proposed to address this power problem. Two promising options are energy harvesting and wireless power transmission (WPT). Energy-harvesting systems convert ambient energy sources, such as light, vibration, thermal, and acoustic sources, into electrical energy. The complete freedom from external electric power sources enables systems to be applied in a variety of environments. However, because of the typically low power densities of the ambient energy sources and limited energy conversion efficiencies, current energy-harvesting systems have been targeting very low-power applications (1 μW-1 mW). Higher output powers are possible by using volumetrically larger energy harvesters, but this becomes size-prohibitive for compact applications. Another disadvantage of energy harvesting devices is that they are at the mercy of the environment. Intermittency and variability of the environmental energy source result in overdesign of the energy harvester to ensure adequate power generation and require more complicated power electronics to regulate voltage and power flow, problems that are exacerbated in unpredictable environments.
In contrast to energy harvesting systems, WPT systems actively transfer power from a source to a receiver, providing deterministic control over the power delivery. WPT relies on power transmission using electromagnetic fields, without requiring physical connections (conductive wire, optic fiber, waveguide, etc.) between the power source and receiver.
As used herein WPT refers to transmission over moderate distances, as opposed to “contactless power transfer,” which generally refers to short-range power delivery across an electrically isolative barrier using transformer cores on either side of the barrier. The most common WPT approaches rely on either radiative electromagnetic waves or near-field capacitive/inductive coupling.
In the radiative electromagnetic wave approach, a focused beam of electromagnetic energy is generated by the source and pointed toward the receiver. For example, a laser can be used as the source, and a photovoltaic material on the receiver can be used to convert the optical energy to electrical energy. One advantage of this “directed energy” approach is that the power can be concentrated in the focused beam, therefore enabling a large amount of power to be transferred in a small area. However, this necessitates knowledge of receiver location and methods for active tracking if the receiver moves in space. Additionally, due the absorptive nature of this radiated energy, the transmission path must be clear of objects, which may be difficult to realize in many applications.
In the near-field capacitive/inductive coupling approach, capacitively or inductively coupled WPT systems transfer power via spatially distributed, time-varying (yet quasi-static) electric or magnetic fields.
Recent research has focused on near-field power transfer using inductively coupled coils. The operating principle of these systems is similar to air-core transformers. FIG. 1 illustrates a basic configuration of the inductively coupled WPT system, which uses two coils 11 and 12 separated by a distance g and functions as a weakly coupled, air-core transformer. Due to the weak mutual inductance between the air-coupled coils, the operating frequency of such systems is usually in the RF range (1-100 MHz) to achieve reasonable efficiency.
One advantage of the capacitive/inductive coupling approach over the radiative electromagnetic wave approach is that power can be distributed over a large volume of space, and arrays of receivers are possible.
Accordingly, the range and transmittance of magnetic fields makes inductively coupled WPT attractive for many applications.
While the inductively coupled WPT systems have the benefit of using magnetic fields to pass through many materials and objects (as compared to electric fields), there are practical limits to both power levels and efficiency, especially for powering wireless sensors.
According to Faraday's law,
                                          V            ⁡                          (              t              )                                =                                    -              N                        ⁢                                          d                ⁢                                                                  ⁢                Φ                                            d                ⁢                                                                  ⁢                t                                                    ,                            (        1        )            the voltage V induced on the receiving coil is proportional to the number of coil turns N and the time-rate-change of magnetic flux Φ through the coil. For a time-varying magnetic field B(t) in a stationary coil, Equation (1) can be rewritten as
                              V          ⁡                      (            t            )                          =                              -            N                    ⁢                                    ∫              S                        ⁢                                                                                ∂                    B                                                        ∂                    t                                                  ·                                                                  ⁢                d                            ⁢                                                          ⁢                              s                .                                                                        (        2        )            For sinusoidal excitation, the voltage is proportional to (angular) frequency, peak magnetic flux density, and the coil area. Because the induced power is proportional to the square of the voltage, maximizing the product of the frequency, peak magnetic flux density, and the coil area will increase the transmitted power.
However, there are strict safety limits on magnetic and electric fields for RF power transmission that greatly restrict the range, efficiency, and thus application of these systems.
As explained above, since the power is proportional to the square of the voltage, in order to deliver a certain amount of power, either the frequency or the flux density in the receiving coil (the area of the receiving coil is usually predetermined) needs to be sufficiently high, which may not be achievable without violating the safety limits.
Specifically, if small coils are used (small area, low number of turns), then the frequency and/or magnetic flux density must be increased in order to increase the power transfer. However, the time-varying electromagnetic fields that permeate the power transmission media can cause safety hazards.
For example, in order to transfer Watts of power in an inductively coupled WPT system, the magnetic flux density that permeates the media may be on the order of 10−4 T. Such strong flux density is only safe when the operating frequency is lower than 100 kHz. For even lower frequencies, the flux density safety limit is higher. For example, flux density up to 10−3 T can be tolerated when the frequency is lower than 760 Hz, and up to 0.4 T of static flux density can be tolerated by general public.
The Institute of Electrical and Electronics Engineers (IEEE) and the International Commission on Non-Ionizing Radiation Protection (ICNIRP) place strict limits on electromagnetic field intensities. For example, ICNIRP data shows that up to 400 mT of static magnetic flux density is safe for the general public to avoid interference with magnetic strips in credit cards or devices such as pace makers. For alternating current (ac) fields, IEEE C95.6 permits up to 1 mT for frequencies below 760 Hz. For frequencies from 760 Hz-100 kHz, IEEE C95.1 restricts the field to 0.1 mT. Accordingly, safety limits and coil size put an upper limit on the power level that can be transmitted via inductively coupled WPT.
Even if the magnetic field densities are kept in a safe regime, the low mutual inductance between the coils generally requires the operating frequency to be high (usually >100 kHz), so that the mutual reactance is sufficiently larger than the coil and radiation resistances. These high frequencies create additional system limitations. First, the high-frequency electromagnetic fields may induce large eddy currents in any conductive materials that are present in the power transmission path. The power loss (and Joule heating) due to these eddy currents is proportional to the square of the frequency. In many home, medical, or industrial applications, power transmission through the metal cases may be required, so eddy current losses will reduce the overall efficiency and may cause unwanted heating problems.
Power transmission efficiency is another consideration. High efficiencies can be achieved for short-distance power transmissions where the resonators are strongly coupled, but the efficiency plummets if the separation distance g is large relative to the size d1 and d2 of the coils 11 and 12 (see FIG. 1). For typical resonators having a Q of 100, the efficiency drops below 50% when the distance is approximately g≈√{square root over (d1d2)}; with a Q of 1000, this distance can be extended by a factor of three. This establishes a fundamental design tradeoff between transmit distance and coil size; longer transmission distances require larger diameter coils or higher quality factors.
Another issue is robustness. To maximize the efficiency and range, the resonators are usually designed to have high quality factors. As a result, the system performance is very sensitive to the resonator parameters, since the transmitter and receiver must be precisely matched. From the manufacturing aspect, the required tolerances in the coil inductance and capacitance are very tight, which can be costly or even impossible to realize. Even if initially matched or manually tuned, uncontrollable parasitic capacitances/inductances, for example due to temperature, humidity, and/or coil positions, can lead to mismatches in resonant frequency. In addition, these uncontrollable parasitic capacitances/inductances can drastically reduce the transmit power and efficiency. To overcome this eventual mismatch, a complicated active tuner/controller is used to compensate for parameter variation. The design of the tuner/controller increases the cost and complexity of the system.
Furthermore, tuning the resonant frequencies of the transmitter and the receiver makes it difficult to power an array of receivers with one single transmitter. Since the high efficiency of the inductive WPT system relies on matching the resonant frequencies of the transmitter and the receivers, all of the receivers need to resonate at the same frequency. This causes interference or cross talk between receivers. That is, the current flow in one receiver may induce significant voltage in other receivers. To mitigate this interference, additional complicated power flow control circuits may be required.
Because of the various limitations on inductive WPT, application of inductive WPT has been limited to short-range, highly specific applications such as electric vehicles and consumer electronic charging pads. While clever electronic control circuits have overcome some of the tuning challenges, there continue to be challenges in widespread implementation of inductive WPT because of the range, power, and efficiency limits of the existing inductive WPT structures.